The oscillatory flow around a spherical object lying on a rough bottom is
investigated by means of direct numerical simulations of continuity and
Navier-Stokes equations. The rough bottom is simulated by a layer/multiple
layers of spherical particles, the size of which is much smaller that the size
of the object. The period and amplitude of the velocity oscillations of the
free stream are chosen to mimic the flow at the bottom of sea waves and the
size of the small spherical particles falls in the range of coarse sand/very
fine gravel. Even though the computational costs allow only the simulation of
moderate values of the Reynolds number characterizing the bottom boundary
layer, the results show that the coherent vortex structures, shed by the
spherical object, can break-up and generate turbulence, if the Reynolds number
of the object is sufficiently large. The knowledge of the velocity field allows
the dynamics of the large scale coherent vortices shed by the object to be
determined and turbulence characteristics to be evaluated. Moreover, the forces
and torques acting on both the large spherical object and the small particles,
simulating sediment grains, can be determined and analysed, thus laying the
groundwork for the investigation of sediment dynamics and scour developments.Comment: 35 pages, 21 figure